


Tail pulling

by silverryu25



Category: Undertale (Video Game)
Genre: Biting, Come Inflation, Drooling, Growling, Knotting, M/M, My First Smut, Plot What Plot/Porn Without Plot, Red is bad at pranks, Sans and Red are not dating, Sanscest - Freeform, Scent Marking, TOTALLY NOT, and Cuteness, be gentle ;w;, doggy position, he always overdoes it, humping, intentionally or not, kitty magic, kustard - Freeform, lots of cum (LOTS XD), pranks gone wrong XD, second chapter tags are, very horny Red, when pranks go wrong
Language: English
Status: Completed
Published: 2019-10-14
Updated: 2020-09-05
Packaged: 2020-12-16 05:02:34
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 9,791
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21030680
Author URL: https://archiveofourown.org/users/silverryu25/pseuds/silverryu25
Summary: Sans keeps teasing Red over acting like a rabid dog so Red decides to get revenge... with a prank... that goes very wrong... or does it? >:3





	1. This is a CATastrophe... right?

**Author's Note:**

> This was inspired by a post made by Muskka that you can see [here](https://twitter.com/Muskka_mu/status/1181617896813977600) X3

Red was sitting on the couch watching TV, trying to relax as usual, but unfortunately for him it just wasn’t working today. 

Usually, this would be his favourite time of day. Him and Sans would relax on the couch, watch a few bad movies (preferably some trash horror films if Red had a choice) and enjoy the quiet while their brothers were out training or doing whatever they did. Red never had quiet time in his universe. Edge was usually on his case all day and never let him really relax. Which made sense given how dangerous their world was, even if Red hated to admit it. That’s why this time of the day was extra important to him now.

When the universes merged, after they got to the surface and everything stabilized, it took everyone a while to get comfortable with the situation. Meeting your alternate selves was… not easy. Especially when you weren’t one who even liked yourself, like Red… or Sans. But somehow, surprising everyone but mostly themselves, Sans and Red found they fit together like two pieces of a puzzle.

Maybe it was that they were both broken in different places, filling up parts of each other that they were missing. Whatever it was, they fit well together and ended up spending a lot of time hanging out, telling stupid jokes and puns, driving their brothers crazy with pranks and japes, drinking condiments (though Red will never understand Sans’ love for that red crap) and just lazing about together.

But that was the extent of their relationship. They were buds, hanging out and joking around. Nothing more. And if Sans sometimes fell asleep on Red, his shirt hiking up, showing those perfect unmarked bones, it wasn’t Red’s fault that he appreciated the sight. It was probably jealousy, right? Because Red’s bones were cracked and marked all over. He kept staring at Sans’ bones only because of that. No other reason… none.

Shaking his head to clear his thoughts from the strange wanderings and focus back on his current predicament, Red sighed like a man cursed. The relaxed atmosphere he was looking forward to was nowhere to be found. Instead, there was a heavy pressure directed at him coming from… Sans… kinda? Well no, it was Sans, just not in his usual form. Which might have been Red’s fault (it was totally his fault).

It started a week ago, due to a fight (all of Red’s problems seemed to come from fights… go figure). Surprisingly, the fight itself wasn’t the problem this time, it was what came after. Sans was teasing him the whole weak about growling like a dog during a fighting match with the underfell dog pack. It wasn’t Red’s fault! That’s the only way that darned pack knows who’s boss! But Sans didn’t care and kept teasing Red for the whole dammed week! He even bought Red a CHEW TOY! A freaking chew toy! (Red laughed over it once the initial indignation passed, but he’d never admit that, not over his dead body). So Red decided a little payback was in order. If Sans was going to tease him about being a mutt, Red wanted to see how Sans would like being teased right back… but as a different animal.

He asked Alphys (the undertale one cause he didn’t like doing business with that crazy bitch from his universe, not even after his Undyne mellowed her out a bit) for help. They plotted a few days and he finally got the means for his revenge! A potion that would show Sans just what it meant to be like an animal. Now he only had to get Sans to drink it… which turned out to be rather easy.

Sans didn’t suspect a thing. They met up at Grillby’s, like usual, and Red already had a bottle of ketchup prepared for him. A special bottle. The potion he got from Alphys thoroughly mixed in so Sans wouldn’t taste it. Even then, Red was sweating bullets, nervous that Sans would end up noticing it. But even if the taste was a little off Sans didn’t say anything, so he drank it. The whole thing. And Red’s grin turned wolfish. This didn’t escape Sans’ notice.

“what’s the wolf grin for?” Sans asked adding with a chuckle, “gotten a good treat today?” 

Red scowled but quickly relaxed, remembering that sweet revenge was in his phalanges already, the grin back full force. “yup,” he chuckled darkly making Sans frown in confusion. “got the best treat. and yer gonna get a better one soon.”

Sans tried to get him to explain but Red wouldn’t budge, so they spent their time in Grillby’s bantering as usual. Right up to the point when Sans gasped and doubled over. Grillby was surprised and ran to help, but Red quickly shooed him away, porting Sans home with an excuse of his counterpart drinking too much.

The moment they got home Red gently lowered Sans on the couch, expecting the effects of the potion to show. The potion was supposed to make Sans summon some ecto cat ears and a tail that couldn’t be unsummoned for a few hours, enough time for Red to tease Sans till he was blue (hah). Except… that didn’t happen. Red fucked up. Big time.

Sans screamed and writhed on the couch, clenching his sternum where his soul was. He was obviously in a lot of pain, no one could act that well. 

“wha’s wrong?” Red panicked and hovered over Sans unsure what to do. “sans? tell me wha’s happenin’?!”

“h-hurts...” Sans managed to press out. “s-soul... *huff*... fee-els tight…”

Red immediately called Alphys and described what was going on, but she had no idea what could be wrong. She told him to bring Sans over if his HP goes down, but if not he should be ok. The potion wasn’t meant to do any permanent harm, only to bring a temporary change to his magic. The effect might have been stronger due to Sans being a skeleton monster, so more of him was dependent on that magic? But nothing could be done now that the process started.

Feeling helpless (and guilty) Red kept careful watch over Sans, looking for any sign of his HP dropping. But it didn’t. Instead, something else happened that made Red’s eyesockets go incredibly round.

Sans shrank. Not a lot, maybe a third... no half his normal size. But he _shrank_, and he grew ears and a tail. Not ecto parts like he was supposed to, but real ears and a tail. They were… adorable. Red shook his head trying to clear that thought out of his skull.

And that’s pretty much how they ended up in this awkward situation. Red sitting on one end of the couch feeling tense (and guilty). Sans sitting on the other side, half as big, wearing only his shirt (because everything else was just too big for his current frame), arms crossed and… pouting. He’d say scowling, but the face he was making was just too cute for that, especially with his ears pressed back and his tail lashing around. Even so, Red couldn’t take it any more and decided to try and apologise… again.

“sans, come on,” Red whined, ironically sounding like a scolded puppy. “‘m telln’ ya it was a mistake. tha’ wasn’t supposed ta happen. ya were supposed ta just get some ecto ears ‘n tail fer a little while. nothin’ like… this.” He finished gesturing to encompass Sans’ whole… situation.

Sans snapped his head towards Red and hissed! He hissed! Red could barely hold back a laugh, quickly covering his teeth to muffle the sound. But Sans’ new ears caught it and his “scowl” deepened.

“no.” was all Sans said and then turned back to the TV, ignoring Red completely.

But Red had had enough and decided to take matters into his own hands… literally. He nudged closer to Sans, ignored the little warning growl and put his (now much bigger) hand on top of Sans’ head, like he would usually put on his shoulder.

“come on buddy, pal, amigo,” Red tried again. “it’s not such a big _cat_astrophe. ya look _purrr_fectly fine like tha’. an’ i ain’t _pullin’ yer tail_.” He joked hoping the puns would make Sans relax, while his hand unconsciously started to rub behind Sans’ ear.

Sans froze up at first, startled when Red put his hand on top of his head, but before he could recover and push Red’s hand off with a huff, Red started scratching behind his ear. An electrifying sensation travelled down his spine from the stop that was being rubbed and Sans started… purring?! The moment he did, Red stopped moving and looked down at Sans, stunned. Sans’ face turned blue and he quickly covered his mouth with both hands, eyesockets blown wide.

“didya just…” Red couldn’t even finish that question because the look on Sans’ face told him everything he needed to know. His grin spread ear to ear (if he had any that is) as he almost purred out “well, well, well. ain’t this _paw_sitively _purr-fect_.”

Sans noticed the glint of mischievous intent in Red’s eyelights and tried to scurry back out of his reach, but he was too slow. Red quickly shoot out a hand and grabbed Sans’ tail.

“nyaaa!” Sans yelled out, then promptly continued to turn the brightest blue Red had ever seen. Letting out a loud laugh Red pulled Sans into his lap and settled back making sure they were both comfortable.

“ya ain’t gettin’ away tha’ easy, sansy.” He drawled out and started to pet and scratch Sans on all the spots he knew cats liked. He thanked the stars for Doomfanger right now. That hellspawn thought him exactly where not to touch and which spots would tame that beast. Since Sans was acting so much like a cat, he just had to focus on all the same spots, right?

He scratched Sans behind the ears, under the chin, petted him along the spine in long strokes that ended at the tip of the tail. And all Sans could do was wiggle and murmur and try (unsuccessfully) to stifle his purring. But the more Red petted him, the more the sounds Sans made - went from denial and rejection to bliss and obvious enjoyment. His eyelights slowly fuzzed-out and got bigger, an obvious sign of relaxation. His body followed, becoming like putty in Red’s lap.

Red was (unexpectedly) really enjoying himself. Sure he was piling up the teasing material, but it wasn’t only that. Seeing Sans melt in his arms made his soul race and a soft blush appear on his cheekbones. He wasn’t sure why but the more he petted Sans, the more he wanted to try and pet him a little more… thoroughly. Like under the shirt.

So when Sans was completely relaxed, Red slowly pushed his hand under his shirt and petted directly down his spine. The moment his hand travelled down, Sans let out a new sound, one that could only be called a moan. But Sans was so blissed out that he didn’t really notice, even if Red did, making the bigger monster freeze when his soul jumped at the sound.

What the hell? Did he just… get aroused at Sans’ moan?

Red’s face turned a brighter shade of his namesake and he didn’t move, trying to figure out what just happened. This made Sans come back a little to himself and he turned to look at Red. Noticing the blush and confused expression on Red’s face he too was confused at first, but then he turned to follow where Red was looking and saw… Red’s hand under his shirt, with Sans’ tail waving happily. Then it hit him. The pleasure and the unrestrained sound he let out. Sans’ blue blush soon joined Red’s as they both stared at each other, neither moving an inch.

It didn’t last. Blame it on Sans’ transformation, his instincts being heightened due to his current form, the beat of his soul as heat spread from the place Red was touching, or whichever excuse he will give himself later for what he did next. Sans looked Red right in the eyesockets, pulled his ears back a little, leaned closer and licked Red’s hand.

It was a kitten lick. Red could barely feel it. But it sent shivers all the way up his arm and straight into his soul. Everything turned red.

The next moment he found himself hovering over Sans, who was pinned between his arms on the couch. Sans’ shirt was completely pulled up, exposing those tiny perfect ribs. His hands were pulled up in a kitten-like pose, while his tail and one of his knees were shyly pulled up over his exposed pelvis. Red hesitated for a second, but one look at Sans’ flustered face, his begging eyelights and his fast breathing, followed by a tiny nod blew all his doubts away.

Red leaned in and placed a hard, needy kiss on Sans’ teeth, his tongue demanding entrance. Sans didn’t hesitate and let Red in to explore his mouth. Red’s tongue dove in conquering Sans’ without much resistance on the smaller’s side. He could taste and feel Sans completely. The little fangs Sans grew due to the transformation made his soul throb. Pulling back to get some air (that they didn’t really need, but habit was a bitch) he looked down at Sans who seemed even more blissed out then while Red was petting him.

“yer gorgeous, sweetheart.” Red blurted out, his mouth faster than his brain.

Sans mewled shyly and blushed. He covered his face with his hands, his ears pressing close to his skull and his tail pressing even harder over his exposed belly.

“none of that.” Red quickly added, grabbing both of Sans’ hands in his larger one and pinning them above his head. Sans’ ears shot up in surprise and he tried tugging his arms free, but they wouldn’t budge. Seeing Sans panic a little Red quickly used his free hand to scratch behind one of his ears while leaning down and whispering into the other “i wanna see ya completely. no hidin’ allowed, kitten.”

Sans shivered, the sensation of one his ears being petted and Red’s deep voice rumbling in his other made electricity run down his spine and magic rush to his pelvis. It didn’t form anything yet, but it was stirred up and made him itch in ways he hadn’t in a long time. His pelvis involuntarily bucked up a little. 

Red noticed, but instead of scratching that itch decided he wanted to play a little more with his kitten before the main event. He nuzzled the side of Sans’ head, near his ear and let out a satisfied rumble, while his hand wandered down to rub and play with Sans’ tiny heaving ribs. Slowly he lowered his head to Sans’ neck, licking between the bones there and leaving a few gentle bites. Enough to sting and leave a mark, but not enough to damage his pets HP. After every bite he followed up with an ardent lick, making sure his magic would seep into the tiny wounds his fangs left on the pristine bones. He could feel Sans’ whole body shiver at every touch, caress, bite and lick. He was so sensitive, so responsive. So perfect.

Sans’ ears were pulled back, his tail still hovered and twitched over his pelvis as if he was trying to use it to stimulate himself. But his control over it was sorely lacking. His eyelights were fuzzed out and unfocused, a soft blush spread on his cheeks. Mouth hanging open, drool leaking out as his breaths started coming out faster. His magic needed more stimulation. He couldn’t take it. He needed more!

“m-more…” he breathed out, barely a whisper.

“wha’s tha’, kitten?” Red asked, his head still buried in Sans’ neck, hot breath ghosting over the sensitive bite marks.

Sans’ breath caught at the sensation, his body arching towards Red. “more!! red more!! please!” He shouted out, with the little breath he had left. His hands desperately struggled in Red’s grip trying to pull Red closer.

“anythin’ fer ya pet,” Red smirked, finally letting go of Sans’ hands, which promptly grabbed his jacket and pulled the bigger monster down for a deep kiss. Red, not expecting such a show of strength from his (currently) tiny counterpart, grunted in surprise. Recovering quickly, he took over the kiss, making Sans moan as he willingly submitted.

Meanwhile, one of Red’s hands was busy playing with Sans’ ribs, while the other went lower grabbing Sans’ tail and gently pulling and petting along its length. The effect was immediate and gorgeous. Sans arched his back, separating from the kiss, and let out a sound that was both a mewl and a moan, his magic finally snapping into place.

Red pulled back, wanting to appreciate everything his pet had to give. Sans’ arched back, the look of almost painful arousal on his face, all his joints aglow with his signature blue magic and a perfect blue gem between Sans’ legs. The soft mound Sans summoned was already glistening with his arousal. Red gulped, his mouth suddenly overflowing with the need to taste that pretty magic.

Without pausing to think, he grabbed Sans’ hips pulling his pelvis up, making Sans squeak in the cutest way, and dove right in. He dragged his tongue over the lips, teasing but not pressing too hard, yet he could feel Sans’ body twitch and shiver. His own magic was starting to ache at this point. It snapped into existence almost at the same moment Sans’ did, and he knew he wouldn’t last long if Sans continued to react like that. Still, he couldn’t just shove in. Sans was way too small for that right now. Red just needed to speed things up a little.

Red used his tongue to spread Sans’ soft lips wide open, his sinuous tongue making it easy to breach Sans’ entrance, while one of his thumbs focused on his pets clit. Red’s tongue twisted around, trying to find Sans’ sweet spot. Once he did, he started a merciless pace of licking and prodding, pressing and twisting with his tongue while focusing on that spot, while following the pace with his thumb. The whole time Sans’ writhed and twisted, mewled and moaned, his breaths short, his face and joints shining blue. And with a final determined plunge of Red’s tongue, Sans came all over Red’s face. Red gently guided him through his first orgasm.

Panting hard as he came down from that blissful high, Sans was gently lovered. He looked up to see Red’s face covered in blue magic, his red tongue licking up the mess. If it was possible Sans swore he could feel his face turn an even darker shade of blue, but he was still too out of it to react any more than that.

Red, panting as well (but for a very different reason), leaned over Sans placing a soft kiss on Sans’ face. “ready, kitten?” He half growled out, obviously having a hard time holding back.

Sans, still out of breath, could only nod as he felt something hard rubbing at his folds. Looking down between them he could see Red had pulled his pants down, just enough to expose his member. It was thick. Normally, Sans would have been able to take it without any thought (even if it would be a tight fit) but at his current size? His bones started rattling a little at the thought. Still, he gritted his teeth and didn’t protest.

Red noticed Sans tensing up and he heard the rattle. Leaning down he licked up Sans’ neck and pet his ear again while rubbing his length over Sans’ soaking folds. “yer ‘kay, pet. ya can take me. ain’t gonna hurt ya. ‘m gonna take it slow. yer gonna be fine.” He whispered between the gentle licks and bites.

The gentle petting of his ears and the deep calming timber of Red’s voice helped Sans relax. The moment he did Red used his chance and started pressing his slicked up girth into Sans’ waiting hole. Being true to his word, he took his time, waiting for Sans to adjust until, finally, Red was in all the way. He paused to kiss Sans, a deep slow languid kiss. A kiss that made sure Sans fully relaxed and Red could move without hurting the smaller one.

Red started a slow pace, pulling out almost all the way, then plunging back in to the hilt. He kept this up as he continued to alternate between kissing Sans and lavishing his neck. His hands roaming over Sans’ ribs, rubbing up and down Sans’ spine while matching his trusts. Soon enough Sans’ breathing had picked up, his hips started moving to meet up Red’s thrusts, his eyelights had blown wide and breaths were coming in short huffs.

Then suddenly he grabbed Red’s shirt and mewled out “mnyyyoore… f-faster… g-go fasterrr. red...”

Hearing his little kitten beg, Red’s control finally snapped and his pace sped up. He pummeled into Sans with a bruising speed that they will both regret in the morning. The new speed, coupled with Sans’ walls constricting around him so deliciously had Red nearing his limit fast.

“*huff* ‘m c-close, kitten.” Red pressed out through clenched teeth, trying to hold out as long as possible.

“n-not yet. more. red *huff* i-i neeeeed m-moooorrrre.” Sans half wailed out.

Red gritted his teeth harder, lifting Sans’ pelvis a little up, trying to get into a position where he could hit that sweet spot better. One of his hands wandered to Sans’ clit again, trying to get his pet closer to completion, while the other went below to support his little mate's (where did that come from? ugh! he didn’t have time to think about that right now!!) pelvis. As Red adjusted his grip, he pressed down on the spot where Sans’ tail connected to his spine, suddenly Sans moaned out and arched his back, his eyelights turning to hearts as his walls clamped down on Red’s dick buried deep inside his pussy.

Red couldn’t take it and with a final thrust, he came, filling Sans up as the smaller one's juices gushed out around his member. Panting hard he gently lowered Sans’ pelvis down and leaned over him, supporting himself on his elbows so he wouldn’t squash the small monster.

It took them both a little to get down from the high and to get their breaths back. Unexpectedly, Sans was the first to recover.

“yo-you planning to pull out or be a _dick_ about it?” His usual cocky grin was back in place.

Red snickered at the dirty pun and fired right back with “not sure. ya were bein’ such a good _pussy_ up ta now.”

They both burst out laughing, the dirty humour not making either of them flinch. Still, Red didn’t want to hurt Sans so he carefully pulled out, making both of their magics dissipate, leaving a warm satisfying feeling behind.

“heh, _good boy_.” Sans said as he pet Red’s head. __

_ _Red (totally not blushing) pushed his tiny hand away and half growled “stop tha’. it’s wha’ got ya in this mess.”_ _

_ _Tilting his head, one ear up while the other flopped a little to the side, Sans just whispered “i know...”_ _

_ _This time Red’s face definitely looked like a Christmas light (it practically lit up the room), but he was too tired and still feeling too high after their little play to deal with this right now. Instead, he quickly pushed up, took his jacket off, wrapped Sans in it like a burrito and laid down with Sans between him and the back of the couch._ _

_ _Sans, not expecting it let out a short indignant grunt (Red would later swear it was a mewl), but didn’t fight the manhandling. When Red settled down, he looked up at him and asked “what the hell?”_ _

_ _“‘ts nap time.” Red mumbled out, ignoring the question completely and already nodding off (a skill he acquired since he started hanging out with Sans)._ _

_ _Sans, knowing he wouldn’t get Red to talk right now, just let out an amused sigh. A nap didn’t sound that bad. Wiggling, he moved up a little, just enough to place his head under Red’s chin. Closing his sockets, a satisfied grin (like a cat that stole the cream) plastered on his face, he nuzzled close and drifted off. If someone was there to notice, they might have heard soft purring coming from the bone pile on the couch. But no one was there, so it probably didn’t happen, right?_ _


	2. Wolf you up

**Summary for the Chapter:**

> After their little kitty adventure in the last chapter Sans and Red finally started dating and even moved in together. Sans even forgave Red for the prank that led to it. Everything was going great for them! Nothing could go wrong now, right?
> 
> Then how could getting drunk with a friend cause this much trouble?

**Notes for the Chapter:**

> HAPPY BIRTHDAY MUSKKA!!! ♥☆*:.｡.o(≧▽≦)o.｡.:*☆♥
> 
> The first chapter was inspired by your incredible art so it was only right that the 2nd one becomes your birthday present ;)  
I hope you enjoy these two dorks going at it again!! XD
> 
> Sorry for the long wait for chapter 2 everyone! But I hope the size of the chapter makes up for the wait at least a little bit UwU

“‘m fiine-e.” The skeleton monster slurred out, waving a hand towards his worried, but no less drunk companion.

“A-are you *hic* sure, Sansss?” His companion slurred back, obviously not believing a word he said. Then again she knew him from way back, so of course, she could see  _ right through him _ (heh, classic).

“‘m fine alph. don’t get your, your… uhhh… your tail in a knot?” Sans shot back, trying to blink himself back to his senses. Stars he really was plastered, wasn’t he? He was starting to see two of Alphys and he was sure he only went to hang out with one… right? Right. No double trouble here. “Hehehe. ‘m hilarious.”

“Ss-sans. You’re talking, ugh, talking to yourself… again.” Alphys mumbled back. Or maybe Sans just couldn’t hear her right? Huh… well who cares.

He had to go home or Red would worry… probably? Maybe? He didn’t tell him after all. Where he was going. Cause (shhhhhh) this was a secret. He came to get… something? Something important? It was-

“But if *hic hic* you’re sure-e.” She cut in, making him lose his train of thought with a frown. “You have the pot-tion, right? *hic*”

_ ‘potion? what potion?’ _ He raised a hand to scratch his skull and hit it with a bag he was holding. Blinking a few times to try and focus on the violent item he realized that must be what Alphys meant… probably. Thinking was hard.

“yep.” He decided to supply, thinking  _ ‘kay, then no use in sticking around’ _ . He could get whatever he should have next time if this wasn’t it. Whatever  _ it _ was. “ ‘m goin’ home. bye alph.”

With a salute, he stepped back through the door and ported in front of his house. It wasn’t one of his best landings, leaving him sprawled out on the front steps, wheezing. Shit, he shouldn’t have done that. His magic was all over the place due to the alcohol, he’s lucky he didn’t end up in a wall (again). Pap would have been so pissed at him. Last time it took them half a day to get him out without taking the house down with him. That was a real character  _ building  _ experience.

“hehehe.” Snickering at his own (dumb) joke, he carefully got up, making sure he doesn’t end up face down again. With shaky steps, he got up and stood in front of the door. A few moments passed with nothing happening, only a few drunken giggles could be heard.

“knock, knock.” Sans said as he knocked, giggling (like a drunk schoolgirl) and waiting for Red to open. 

But several minutes passed and nothing happened. Sans leaned his skull on the door, frowning as he tried to think.

_ ‘red… wasn’t… home? he was. he was somewhere. he said he was gonna be… work! yes. work. an’ i gotta be... sneaky. okay. sneaky. gotta get home before red comes back. wait. this is home. good job. hehehe.’ _ With that line of thought (if you can call it that) he finally reached for his keys.

Clack. Clack. Scrape. Clack.

“hehehe. stop movin’.” Blinking a few more times to focus, with a triumphant “aha!” he finally managed to get the key into the lock and open the uncooperative door.

Stumbling inside, he quickly glanced around. Good, he was alone. With a few more sneaky giggles, he went to the kitchen to get some water. The alcohol was fun and all but, stars, he was thirsty after drinking so much. It took him a little manoeuvring but he finally got to the sink, and as he tried to open the faucet something got stuck. A bag.

Staring at it uncomprehendingly for a moment, what was… with a soft “oh!” and a hit to his forehead, he remembered. It was what Alphys gave him. That’s why she called him over. Well, she didn’t tell him that was the reason.

She called him over for anime night, and since he was alone for the night (and kept refusing her before) he decided to go. But once he realized Alphys wanted to watch some kitty anime he outright refused.

No thank you. No cats. Ever. No. Just… not after  _ that _ .

Red’s little jape on Sans’ expense was more than enough to swear him off cats forever. At least the anime kind. Nope. Not again.

Instead, they decided to talk. And drink. And talk and drink some more. This went on for a while, and when Alphys’ stammering started becoming slurring, she told Sans she prepared something for him. It was some kind of potion. Something to do with revenge for the prank Red did? Something that would “bring out the real Red”. It would… do something. But no matter how hard Sans thought about it now he couldn’t remember what Alphys meant. He remembered thinking that he probably won’t use it cause it sounded suspicious, but she wouldn’t take no for an answer. So he took it and decided to throw it out? Or hide it?

With a frown, he put the bag with its mystery contents on the counter, then let the water flow and showed his skull under it. He had to sober up a bit, his skull was way too fuzzy and slow.

A few minutes later, he straightened up and wiped his skull dry. He could see a little better, the whole room wasn’t spinning as hard. But he still felt pretty out of it.

_ ‘ketchup. that’ll help.’ _ His buzzed skull supplied and he staggered his way to the refrigerator. Rummaging around he finally located one bottle and took a few deep swings of its precious content.

“ahhhhh. that hits the spot.” With a satisfied sigh, he closed the door of the fridge and turned around. He was supposed to do something? Something important.

Welp, no use in worrying. Whatever it was it could wait for morning. He was way too drunk and tired to care, and it probably wasn’t so urgent. Yeah. It’ll probably be ok.

… right?

With a final shrug, he made his way to his and Red’s room, already half asleep as he stumbled up the stairs.

\----

“fuck. shit.” Swearing like a drunken sailor (though he was unfortunately completely sober) Red ported in front of his house. “fuckin’ hope that  _ shithole  _ burns down.”

He was (very obviously) in a  _ very  _ bad mood. His whole day was hell. Working at that place full of judgmental assholes wasn’t doing anything for his anger issues, and swearing could help him burn only burn so much steam… even if it was probably the better option. The other one included shanking a few of the idiots that made his day such a fucking nightmare. He really needed to find a different job, before he did something he’d regret.

With a scowl, and more swears uttered under his breath, Red got to the front door as he dug the keys out. But as he tried the doorknob he noticed the door was unlocked, making his scowl deepen. He knew this universe was safe, he did, but he hated how careless Sans got sometimes. If it wasn’t so late he would give that lazyass a piece of his mind. But it was. And he was tired. So fucking tired.

Instead, he passive-aggressively slammed the door shut, hoping (but doubting) he’d at least make Sans flinch, and made sure to lock it. That fucking vanilla dumbass is gonna get himself killed one day. And if that happens Red’s gonna kill him again, just because. He had the right now that they were officially dating after all.

Pulling a hand over his face, he decided he needed a drink. Now. There was nothing some good old mustard can’t solve. It always helped him calm down. Reminded him of the only safe place he had in his old underground. The place where he could relax and let his guard down, knowing Grillbz would skin anyone who even thought of trying anything in his establishment. So it became his comfort drink in a way.

Going into the kitchen he zeroed in on the fridge, intent to get his prize. He reached into the usual spot where he stashed his mustard but there was nothing. His scowl turning sour, he stuck his skull into the fridge digging around old ketchup bottles, chips packets, old pasta and lasagna dishes but… nothing. There was no mustard left.

“fuckin’ shit! ain’t dat just fuckin’ perfect.” He slammed the fridge door closed, making the whole thing shake in place, and with a growl turned around. What the fuck does he do now? He won’t be able to sleep without something to take the edge off.

As he raged over the lack of mustard he noticed a bag on the counter. It didn’t look like it had much in it, but maybe (angel dammit, maybe) Sans got some mustard for him and left it out. It would be warm left out like that, but hell he’d drink it anyway. 

With quick steps he tore the bag open and… there it was. A mustard bottle!

“thank fuck!” He half yelled out as he held the bottle, hands shaking a bit. He knew that he was overreacting, but who cares. There was no one to see him here.

The bottle was a bit weird. There wasn’t a label on it, though it did look exactly like the usual brand he drank. With a shrug (fucking factory error probably), Red uncorked the bottle and drank half in one go.

“hell yea. dat hits da spot.” With a satisfied sigh and half lidded sockets, he let the taste linger on his tongue. It was a bit… weird. The usual taste had something strangely bitter to it, but it wasn’t bad. He kinda liked it. So he downed the rest, tossing the bottle over his shoulder into the trash.

Satisfied, he decided it was time for his well earned nap. Tomorrow he’d see about getting a different job, but today he’d go and  cuddle sleep with his bonefirend (thinking about this still made him blush a bit, though he’d never admit it). With a scratch to his sucrum, he made his way up the stairs, not noticing the little rumble his soul gave.

\----

“mhnmm.” The skeleton let out a half awake sound, shifting a little.

It was… hot. Very hot. He didn’t usually feel temperature that much, but he could feel something big and hot pressing on his back. Reluctant to wake up completely, he used one of his hands to feel around him, but he couldn’t move much. He was surrounded, a body pressed to his back, arms wrapped around him, legs tangled up. And there was something… fluffy? Something fluffy pressed over his hips.

Wiggling his hand free of the arms that were hugging him close, he tried to figure out what that fluffy thing was. Maybe Red left his jacket on the bed again and Sans got tangled up in it? As his phalanges moved down he carefully felt the fluffy thing. It was soft to the touch. Very soft and silky. Kinda nice. Deciding to get a better look he grabbed it and tried to pull it up. Instead, he only managed a slight tug, not moving it from place, followed by a soft growl.

Huh? Growl?

Giving a few more gentle tugs, Sans realized that whatever the fluffy thing was, it wouldn’t budge. With a defeated sigh, he decided to open his sockets to see what was happening. It took him a few moments to focus his eyelights, no doubt because of the leftover alcohol in his system. He wasn’t feeling the full effects yet, but he could already tell that he was gonna regret thee drinking in the morning.

Finally focusing his eyelights, he looked down and froze. The arms around him were… huge. Well not Asgore huge, but definitely bigger than Papyrus’ arms were. And  _ most definitely  _ bigger than Red’s. Red was only a little bigger than Sans, maybe an inch or two. But these? Even lying down he could tell the monster holding him was at least twice his size.

His breath hitched. This was bad. Really bad. If Red comes home and finds him like this, with another monster in their bed, shit would hit the fan. He wasn’t sure he could stop Red from doing something they would both regret.

But who the hell is that? Sans didn’t remember much of the night but he knew he came home alone. So who was in his and Red’s bed, engulfing Sans’ form completely?

But first, Sans had to get out. Now. He’d figure everything else out later.

Wiggling his body slowly, trying to free his arms and legs so he could slip out (since teleporting was impossible while someone is holding onto him) he tried to be as gentle as possible, trying to avoid waking the other. He had to go slow. Gentle. Just a little bit more and-

Suddenly there was a growl next to his skull and the grip around him tightened. He froze. Didn’t even breathe, praying he didn’t mess up and wake the sleeping beast. Cause that was what the one at his back sounded like. A beast. That growl was deep, and even if it sounded more annoyed than anything else, Sans didn’t want to risk it.

He waited, listening to the other’s breathing, waiting to make sure the other was completely asleep. It took a few tense minutes, minutes that felt like hours, but finally the growling stopped and the beast’s breath evened out. With a sigh of relief, Sans decided to try one more time. Slowly and gently. He moved his arms and legs.

Finally, feeling triumphant, he freed one of his legs. Not wasting any time, he quickly focused on freeing the other. Unfortunately for him, the movements made his pelvis press back into the beast that was still hugging him close. He didn’t notice. So he did it again. And again. And again.

Since Sans’ whole focus was on freeing himself, he didn’t notice that the one behind him was slowly starting to breath harder, his body slowly heating up even more. His breaths gradually grew into ragged pants, body curling up around Sans, making the smaller monster freeze again.

Sans could feel the hot breath on the back of his vertebrae, making his bones heat up even more. The beast behind him started moving, making Sans think he was waking up. But no. It was worse. The beast started moving his hips, humping Sans from the back.

With a mortified blush, Sans quickly pressed a hand over his mouth to stifle any sounds. His body was betraying him. He could feel his magic react to the stimulation, even though he didn’t want it. And he couldn’t understand why it was happening. He didn’t really react like this to anyone but Red. Especially not since they started dating. He could feel the panic rising and his soul squeeze, thinking he was betraying-

“ngh. saaaaans.” The moan behind his back made him freeze, eyelights gone. He knew that voice! Sure it sounded a little deeper than usual, with a growly quality to it that it didn’t usually possess. But he knew that voice.

“r-red?” He half whispered, unable to wrap his head around what was happening. Finally gaining the courage, he turned around, catching a glimpse of the beast hugging and humping him.

It was Red. And yet it wasn’t?

He was bigger. A  _ lot  _ bigger. And he had… ears? What the hell?

Slowly Sans tried to see more of Red, twisting his neck to look down. He managed to see that Red could barely fit on their bed at the moment, his leg (the one not wrapped around Sans) hanging over the edge. And there was something waiving behind him. Sans couldn’t be sure what it was but… the fluffy thing that was on Sans’ hips before was gone, so that was probably a tail?

Sockets wide, Sans laid back down and stared in front of him. He was trying to wrap his skull around this, completely lost on what was happening. It was like Red transformed into some kind of half skeleton, half dog monster? Or was it half wolf? Hard to tell from this position. But how could that be? He was alright this morning when he went to work? Did something happen there or-

His thoughts were suddenly interrupted with a hard push from behind, Red humping him harder than before, breath quickening. Sans let out a surprised yelp, quickly covering his mouth. He could see his own blush in the dark room. Fuck that felt good.

Now that he knew it was Red doing it to him, he could feel his body responding even more, the anxiety and panic completely leaving him. He wasn’t sure what happened, but he wouldn’t complain for now. Instead, he started gently rocking back into Red’s movements, feeling his magic start to pool in his pelvis.

Stars, that felt so good.

Red was all around him. Completely engulfing his form, his huge arms pulling Sans tight to the broad chest, which he could feel rising and falling in quick breaths. It was… nice. He felt protected. Safe.

Red let out a growl, leaning his head down to Sans’ neck and pressing his teeth to the vertebrae there. Sans’ breath hitched at the action, prompting another, possessive, growl from Red.

On the next hump, he could feel something warmer pressing to his pelvis. Red’s magic. It was hot. And it was  _ big _ .

Red was usually pretty girthy, but not too long. But this? This was big. He could feel it press from the tip of his sacrum all the way up to his spine, a fair bit over his iliac crests. He gulped wondering how that would feel buried deep inside him. That thought made his magic spark and, almost painfully quickly, snap into an already dripping wet pussy.

Stars, why was this turning him on so much?

Suddenly he heard Red take a deep breath as he pressed his nasal cavity into Sans’ shoulder. Was he sniffing him? Despite the incredulity over being benignly sniffed, it made Sans shiver and wonder if Red liked what he smelled.

Sans felt Red’s tongue press a long line all the way from his collar bone, up his neck and to the bottom of his jaw, making Sans yelp and jump. He could feel his blush spread down the same line Red’s tongue took.

“mmmmmm. tasty.” Red growled out as he pressed his teeth to the back of Sans’ neck, making Sans shudder as he felt those fangs on such a vulnerable spot. “an’ ya smell soooo good sansy.”

Red didn’t pause the humping as he talked, making Sans gently rock in his arms. He continued nuzzling the back of the now smaller monster's neck, still half asleep. Then his hands started wandering, feeling the need to get closer to Sans. He could feel his own body being hotter than usual, but he wrote it off as part of his arousal. 

Still… something felt strange. Sans felt different? More fragile? Kinda similar to… that time. Their first time. When he drank that potion.

Forcing his sockets open (why did they feel so heavy?) he looked in front and there was Sans. In his arms. Barely half Red’s size. What the fuck?

“sans?” He growled out, coughing to try and clear his throat. Why did his voice sound deeper? “why are ya so samll?”

Sans turned his head, a shocked expression on his face, like he didn’t understand the question. His blush deepening, Sans gave a little frown and said “it’s not me red.”

“huh?” Red didn’t understand what Sans meant so he (very reluctantly) stopped the rocking of his hips and raised himself on one elbow. Then looked down. 

He was… big. A lot bigger than usual and, noticing a new weight at his backside, he used his free hand to pull something up. “a tail? wha’ da hell? sans wha’ da fuck is goin’ on?!”

“i-i don’t know. i thought you would know? didn’t you come home like this?” Sans’ voice sounded sincere, and Red couldn’t smell any falsehood on him (since when the fuck could he do that?), so he relaxed a bit.

“no.” He said softly, then felt a soft growl escape him. Well this was fucking weird but he didn’t really feel bad. “do i look a lot differen’?”

“not a lot. y-you just got ears and a tail like a dog? or wolf?” Sans managed with a frown. A memory of something… important was surfacing but he couldn’t focus enough on it. Not with his magic still demanding all his attention. Gulping heavily, trying to focus, he added “how you feelin’?”

“uhhhh…” He felt… horny. Very,  _ very  _ horny. His bones were heated up and his magic was aching. It was never this bad before. “fine. hot. an’...”

“and?” Sans hesitatingly prompted.

“an’ like i really wanna  _ wolf  _ ya up.” Red practically growled out. But the growl quickly turned into a whine, making Sans jump. The smaller skeleton quickly turned around trying to see what upset Red, only to be met by a bright red blush, and a predatory grin, expanding on the scarred skull.

The whine was followed by Red grabbing Sans by the waist and lifting them both up, making Sans let out a yelp (it was actually more of a squeak, not that he’d ever admit it). Without pause, Red took off all their clothes, having no patience for any teasing. And, suddenly, Sans found himself on his arms and knees on the bed. Red looming behind him.

“red, what-” Sans was cut off when he felt Red lean over him, pressing his large ribcage to Sans’ spine, his weight pressing Sans down a little. Red was heavy. Heavy enough to pin Sans in place. He wasn’t going anywhere soon.

“i need ya sansy.” Red panted out, his tongue hanging out, drool dripping to the back of Sans’ neck, making shivers run down Sans’ spine. 

Red sounded like he was at his limit already, not that Sans minded. His magic had been summoned and ready for a while already, and the Red manhandling him out of his clothes didn’t help cool it off at all. He was more than ready and willing. When he felt Red press his cock to his sacrum, his breath hitched and his pussy clenched. He needed to be filled, fast.

With a jerk of his own hips Sans pressed back into Red’s rocking, trying to get Red to slip between his lips. With a growl, that was a lot deeper and more threatening than any he’d let out until now, Red used one of his hands to stop Sans’ hips from moving. His grip was almost bruising, and Sans was sure there would be a mark there tomorrow.

Suddenly, the growl still going, Red pressed his teeth to the back of Sans’ neck and then…  _ bit _ down. Sans’ back arched as he tried to both get away and get closer to Red. He could feel those sharp teeth penetrate his vertebrae, feel the marrow slip through the cracks Red’s teeth made. And all he could do was let out a silent scream as his eyelights blew wide, making his vision fuzzy. He could swear he saw stars.

When Red pulled the teeth back and licked over the bite mark he left, Sans’ whole body went limp. His hands gave out and he fell forward onto the pillow, making Red follow to press even harder into his spine, pinning Sans’ completely.

Red pressed the side of his skull to Sans’, rubbing and nuzzling his cheek over his mate. Marking him. Making sure Sans would smell completely like Red. Everyone will know Sans belongs to  _ him _ .

Sans let it all happen, he was already overwhelmed and Red hasn’t even touched his summoned magic. He needed Red so badly.

“p-please red. please. pleas-se. r-red.” With a soft breathy whine (stars, now  _ he  _ sounded like a needy puppy), Sans started mumbling out, trying to rock his hips as much as Red let him. Which wasn’t much.

It only earned him a warning growl, and a tighter grip on his iliac crest. But Red was getting close to his limit as well so he decided to be merciful. Still, Sans was so much smaller than him right now, he had to prepare him at least a little bit. So he let go of Sans’ hip and used his fingers to rub along Sans’ lips, making sure to collect as much slick as he could. When he was sure he wouldn’t hurt Sans, he pressed one finger into his waiting hole and used his thumb to rub Sans’ clit.

Sans arched his spine as he felt the finger enter him. It was big. Bigger than usual. As big as it was their first time. The burn felt so good, especially with his clit getting rubbed in teasing strokes and swipes at the same time. Soon enough Sans’ breaths came out even faster, his hips squirming, trying to get Red to do more.

Red didn’t need any more invitation than that. He quickly plunged in another finger, spreading Sans wider and reaching deeper. He didn’t pause the attention on Sans’ clit, making sure Sans would relax enough so he could keep going. He kept licking Sans’ neck and leaving smaller bite marks all over his neck and collarbones.

All of it left a panting and squirming mess. Sans His pussy was squeezing Red’s fingers tightly, the sensation pulling out sweet moans and hiccups out of Sans. Then Red hit a bundle of nerves, making Sans shriek as his walls clenched tight and he came. Red continued to stroke Sans through the orgasm, making sure his mate got as much pleasure as possible out of it.

When Sans finally came down he was breathing hard, blush spreading from his cheeks down to his shoulders, sweating, eyelights blown wide. Red could only stare.

“beautiful.” He murmured, unsure if Sans even heard him. Uncaring, he leaned down, turned Sans’ head towards him, and kissed him. He licked Sans’ teeth to ask for permission, and it was enthusiastically given.

The kiss was deep and lasted long enough to leave them both out of breath as they broke it off. Sans’ eyelights were shaped like fuzzy hearts and Red had a feeling his were doing the same thing. Sans was just so gorgeous when he looked like this, wrecked and completely submitted to Red. Perfect.

“ready sansy?” Red asked as he licked over Sans’ mandible, cleaning him off from the spit that slipped through Sans’ teeth during the kiss. Fuck, he tasted so good.

“mhm.” Sans mumbled up, pressing his skull into the pillow below him.

“use yer words, sweetheart.” Red growled out.

“yes. ‘m r-ready. please-se red?” Sans stammered out, voice sounding half broken.

With a pleased smirk Red pulled the smaller monster’s hips, holding them with one hand, still pressign Sans ribs and head into the bed. He used the other as support for himself shifting it closer to Sans’ skull. When he found his balance he turned Sans’ head back so he could kiss him.

As Red deepened the kiss he rubbed his member along Sans’ soaked lips, making sure to lube himself up properly with Sans’ release and his own precome. He was close. All the smells and sounds making it hard to hold himself back like he usually did. Hopefully, he could get Sans off one more time before he lost control.

He could feel they were both ready. Breaking the kiss, Red put his teeth around to the back of Sans’ neck again, gently biting down, he pressed inside. As the head of his cock penetrated Sans’ entrance Sans let out a sultry moan, his pussy clenching around Red in anticipation. The combination of everything, Sans’ smell, the sounds he was making, his pussy trying to pull Red in, the taste in his mouth, it was all too much. Red lost the final shreds of his control and plunged in in one hard push. Sans screamed out with what was left of his voice, seeing stars he arched his spine up and pressed closer to Red.

Red was big. And girthy. And Sans felt so full, almost too full. His walls strained to take in the huge member that pressed into his deepest parts. He was bordering between pain and pleasure for a moment, breath coming in short panicky gasps. Stars that felt so good. He could get addicted to this.

Without giving Sans much time to adjust Red started up a harsh pace, pulling out until only the tip of his cock was inside then plunging back in. The pace rocked Sans’s smaller body back and forth with every movement, making him let out soft whines and huffs of breath, his voice practically gone.

This was pure heaven. Red could feel Sans’ walls slickening even more, and pulling him in deeper and deeper. And soon enough he could feel a pressure at the base of his cock. He didn’t know what it was but he knew he would come soon. With a deep groan, his teeth still pressed to Sans’ neck making the smaller skeleton shiver as the sound reverberated through his bones, Red moved the hand on Sans’ hip to rub his clit. He would make sure Sans came again, dammit.

Red could feel his pace start to stutter, so he doubled his attention to Sans’ clit trying to get him over that blissful edge. He knew Sans was close, he could feel his walls flutter around his cock as it pressed deep into the tiny monster. Yet he knew Sans could take more. So he kept the pace, pressing in deeper and deeper with every rock of his pelvis. Hunting that final moment of pleasure for them both.

With an almost frustrated growl, he turned Sans’ head towards him and kissed him hard, pressing his clit at the same time. Sans released a stuttered moan into the kiss as he came, his walls clenching hard, squeezing Red’s member, pulling him in. Red pressed in one final time, and then felt his cock expand at the base. Before he could even think about what was happening he felt himself coming. And coming. And coming even more. There was so much! And every spurt felt like pure heaven.

Sans felt himself being filled up, his pussy already overflowing, but the cum couldn’t go anywhere. Something was blocking his entrance. With a fuzzy mind he realized Red had  _ knotted _ him. There was a moment of panic about where all the cum would go, but his body reacted instinctively. It formed an ecto belly from his pelvis to his ribs, along with a womb to take the cum in. The moment it formed he let out a sigh of relief as he felt the hot cum flow from his overfull pussy to the newly created space. The relief didn’t last that long as he felt his ecto belly slowly expanding.

With a soft whine as Red released him from the kiss and Sans risked looking down. Fuck. His belly was swollen (it was starting to hang low towards the bed) and, he could see the red cum through the ecto flesh. There was a lot, and more was coming. He started squirming a bit, unsure if he could take more. He was already overstimulated and the feeling of Red’s cock twitching inside him as he released more and more cum into him was starting to feel uncomfortable.

“shhhh. sansy, calm down. it’s gonna be ‘kay.” Red shushed him, laying them both on their sides, still buried deep in Sans and still coming. “i can’ stop it, but it’s gonna be ‘kay.”

Sans shivered, feeling slightly better now that he was on his side. His belly didn’t feel as heavy in this pose, though he still felt the heat and pressure as it expanded. Hopefully it would stop before he reached his limit.

“‘kay.” He managed to croak out, his voice feeling hoarse from all the moaning and screaming. He’ll probably have to call in sick tomorrow. Not that he’ll be lying about it much. He probably won’t be walking for awhile after tonight anyway.

With a soft sigh, he cuddled back into Red, reaching one hand to pet the bigger monster’s skull and ears. A soft sleepy smile spread on his skull, eyesockets half lidded. The ears were fluffy and nice to pet.

Red nuzzled into Sans’ neck as the smaller one pet him. He could feel Sans touching his ears, which made them twitch a little, but didn’t pay much attention to it. It felt nice and he'd worry over it in the morning. For now, he was going to enjoy the feeling of pride and satisfaction over taking such good care of his small mate. Wait… mate? Since when did he think of Sans as his mate?

Filing that away for later, he took Sans’ hand and left a few soft skeleton kisses on his palm. He felt so good and he wanted it to last forever.

Sans let out a soft giggle at the kisses, blush sparking back up.

“you kno’ this isn’t fair, right?” Sans said, feeling sleep gently drag him under.

“wha’ ain’t fair, sweetheart?” Red asked, letting go of Sans’ hand he gently placed his own over Sans’ belly looking for any discomfort on Sans’ face. Seeing none he started gently rubbing and petting over it. Feeling the swell and warmth. Fuck, that was really hot.

“you bein’ bigg’r again.” Sans mumbled out, making Red strain to understand him.

“heh.” Red snickered out, continuing to pet Sans as a smug grin spread on his teeth. "wha' can i say sansy, life’s a  _ bitch _ ."

**Notes for the Chapter:**

> Welp! Can I just say that karma is a _bitch_? XD  
Backfiring pranks are my favourite and even if Sans wasn't the one cooking this one up, he still doomed himself by accepting that potion from Alphys ;)
> 
> Alphys really outdid herself this time though. The potion had none of the nasty side effects like the last one (the transformation didn't hurt so Red didn't wake up) and it brought some interesting new ones (Red feeling heated up was one XD). Now the question is will the new potion wear off? Or will there be some side effects~ >:3c
> 
> That's for you to decide though! ;D

**Author's Note:**

> These two dorks were totally dating even if they didn't wanna admit it... everyone else thought so XD
> 
> The love was there even if they acted like "buds" and they only needed a push to finally drop the act and fess up. And this was a purrrrfect push ;)
> 
> Let me know what you think! I hope I got the kitty behaviour down and I hope you enjoyed these two silly dorks first time X3


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